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<tr class="t-nv"><td colspan="5">                              </td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="Integral.html" title="cpp/experimental/ranges/concepts/Integral"><span class="t-lines"><span>Integral</span></span></a></div></div></td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>Assignable</span></span></strong></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="Swappable.html" title="cpp/experimental/ranges/concepts/Swappable"><span class="t-lines"><span>Swappable</span><span>SwappableWith</span></span></a></div></div></td></tr>
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<tr class="t-nv-h2"><td colspan="5"> 对象概念</td></tr>
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<tr class="t-nv"><td colspan="5">                              </td></tr>
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<tr class="t-nv"><td colspan="5">                              </td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="Semiregular.html" title="cpp/experimental/ranges/concepts/Semiregular"><span class="t-lines"><span>Semiregular</span></span></a></div></div></td></tr>
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<tr class="t-nv"><td colspan="5"> <br>
</td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 比较概念</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="Boolean.html" title="cpp/experimental/ranges/concepts/Boolean"><span class="t-lines"><span>Boolean</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <br>
</td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="WeaklyEqualityComparableWith.html" title="cpp/experimental/ranges/concepts/WeaklyEqualityComparableWith"><span class="t-lines"><span>WeaklyEqualityComparableWith</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <br>
</td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="EqualityComparable.html" title="cpp/experimental/ranges/concepts/EqualityComparable"><span class="t-lines"><span>EqualityComparable</span><span>EqualityComparableWith</span></span></a></div></div></td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="StrictTotallyOrdered.html" title="cpp/experimental/ranges/concepts/StrictTotallyOrdered"><span class="t-lines"><span>StrictTotallyOrdered</span><span>StrictTotallyOrderedWith</span></span></a></div></div></td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 可调用概念</td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="Invocable.html" title="cpp/experimental/ranges/concepts/Invocable"><span class="t-lines"><span>Invocable</span><span>RegularInvocable</span></span></a></div></div></td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="Predicate.html" title="cpp/experimental/ranges/concepts/Predicate"><span class="t-lines"><span>Predicate</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5">                                        </td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="Relation.html" title="cpp/experimental/ranges/concepts/Relation"><span class="t-lines"><span>Relation</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5">                              </td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="StrictWeakOrder.html" title="cpp/experimental/ranges/concepts/StrictWeakOrder"><span class="t-lines"><span>StrictWeakOrder</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <br>
</td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> URNG 概念</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="UniformRandomNumberGenerator.html" title="cpp/experimental/ranges/concepts/UniformRandomNumberGenerator"><span class="t-lines"><span>UniformRandomNumberGenerator</span></span></a></div></div></td></tr>
</table></div></td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>
<tr class="t-dsc-header">
<td> <div>定义于头文件 <code><a href="../../../header/experimental/ranges/concepts.html" title="cpp/header/experimental/ranges/concepts">&lt;experimental/ranges/concepts&gt;</a></code>
 </div></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl t-since-ranges">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span> <span class="kw1">class</span> T, <span class="kw1">class</span> U <span class="sy1">&gt;</span><br>
<p>concept <span class="kw4">bool</span> Assignable <span class="sy1">=</span><br>
  <a href="../../../types/is_lvalue_reference.html"><span class="kw464">std::<span class="me2">is_lvalue_reference</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span> <span class="sy3">&amp;&amp;</span><br>
  <a href="CommonReference.html"><span class="kw2533">ranges::<span class="me2">CommonReference</span></span></a><span class="sy1">&lt;</span><br>
    <span class="kw4">const</span> <a href="../../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span>,<br>
    <span class="kw4">const</span> <a href="../../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="sy1">&gt;</span> <span class="sy3">&amp;&amp;</span><br>
  requires<span class="br0">(</span>T t, U<span class="sy3">&amp;&amp;</span> u<span class="br0">)</span> <span class="br0">{</span><br>
    <span class="br0">{</span> t <span class="sy1">=</span> <a href="../../../utility/forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="br0">(</span>u<span class="br0">)</span> <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> <a href="Same.html"><span class="kw2530">ranges::<span class="me2">Same</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;&amp;</span><span class="sy4">;</span><br>
</p>
  <span class="br0">}</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-ranges-ts t-mark-ts">(范围 TS)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>概念 <code>Assignable&lt;T, U&gt;</code> 指定类型和值类别为 <code>U</code> 所指定的表达式能被赋值给类型为 <code>T</code> 所指定的左值表达式。
</p><p>给定
</p>
<ul><li> <code>t</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span></span></span> 类型的指代对象 <code>o</code> 的左值，
</li><li> <code>u</code> ，使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>u<span class="br0">)</span><span class="br0">)</span></span></span> 为 <code>U</code> 的表达式，
</li><li> <code>u2</code> ，等于 <code>u</code> 的另一对象。
</li></ul>
<p><code>Assignable&lt;T, U&gt;</code> 得到满足，仅若
</p>
<ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../../memory/addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span>t <span class="sy1">=</span> u<span class="br0">)</span> <span class="sy1">==</span> <a href="../../../memory/addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span>o<span class="br0">)</span></span></span> （即赋值表达式产生指代左运算数的左值）；
</li><li> 求值 <span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy1">=</span> u</span></span> 后：
<ul><li>  <code>t</code> 等于 <code>u2</code> ，除非 <code>u</code> 是指代 <code>o</code> 的非 const 亡值（即赋值是自移动赋值），
</li><li> 若 <code>u</code> 为泛左值：
<ul><li> 若它是非 const 亡值，则其所指代的对象在合法但未指定的状态；
</li><li> 否则，不修改其所指代的对象；
</li></ul>
</li></ul>
</li></ul>
<p><code>Assignable&lt;T, U&gt;</code> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../../types/is_lvalue_reference.html"><span class="kw464">std::<span class="me2">is_lvalue_reference</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span></span></span> 间不必有任何蕴含关系。
</p>
<h3><span class="mw-headline" id=".E7.9B.B8.E7.AD.89.E6.80.A7.E4.BF.9D.E6.8C.81">相等性保持</span></h3>
<p>若表达式对给定的相等输入产生相等输出，则它<i>保持相等性</i>。
</p>
<ul><li> 表达式的输入由其运算数组成。
</li><li> 表达式的输出由其结果和表达式所修改的所有运算数（若存在）组成。
</li></ul>
<p>进一步要求每个要求保持相等性的表达式都<i>稳定</i>：这种表达式带相同输入对象的二次求值必须拥有相等的输出，而无任何对这些输入对象的显式中间修改。
</p><p>除非另外提醒，每个用于 <i>requires 表达式</i>中的表达式都要求保持相等性且稳定，而表达式的求值必须只修改其非常运算数。必须不修改常运算数。
</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span> expression <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> Same<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;&amp;</span></span></span> 形式的推导制约等效地要求 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>expression<span class="br0">)</span><span class="br0">)</span><span class="sy3">&amp;&amp;</span></span></span> 为与 <code>T&amp;&amp;</code> 准确相同的类型。这会制约表达式的类型与其值类别。
</p><p>赋值不需要是全函数。特别是，若赋值给某对象 <code>x</code> 能导致修改某个其他对象 <code>y</code> ，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">x <span class="sy1">=</span> y</span></span> 可能不在 <code>=</code> 的定义域中。若右运算数直接或间接地为左运算数所占有（例如用基于结点的数据结构中的指向结点的智能指针，或用类似 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../../container/vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><a href="../../../utility/any.html"><span class="kw1071">std::<span class="me2">any</span></span></a><span class="sy1">&gt;</span></span></span> 的构造），则这通常会发生。
</p>
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